The present invention relates to an engine fuel control device and an air-fuel ratio control method during idling and, more particularly, to an improvement made on an air-fuel ratio control method performed by a fuel control system that supplies the engine with gaseous fuel during idling when the engine is started.
A gaseous fuel vehicle mounted with an engine operating on CNG (compressed natural gas), a type of gaseous fuel, is known. The gaseous fuel in a gaseous fuel container is taken through a fuel supply pipe. A pressure reducing valve then regulates a pressure and a flow rate of the gaseous fuel to corresponding predetermined levels. A gas mixer finally mixes the gaseous fuel with air and the fuel is supplied through a fixed venturi to the engine.
Japanese Patent Laid-open No. 2000-18100 discloses a fuel supply system for a gaseous fuel engine. A gaseous fuel supply system disclosed in this publication has the following arrangement. Namely, a three-port solenoid valve is provided at a place near a fixed venturi of a gas mixer located in a point midway a fuel supply pipe. There is also provided a bypass passage that connects the three-port solenoid valve to an air intake system located downstream from a throttle valve of the engine. A control means is provided for controlling the position of the three-port solenoid valve, thereby directing the gaseous fuel toward a side of the bypass passage. In addition, a branch pipe that branches from the fuel supply pipe downstream from a pressure reducing valve. The branch pipe is connected to an auxiliary injector disposed in the air intake system downstream from the engine throttle valve. There is provided the three-port solenoid valve at the place near the fixed venturi of the gas mixer located in a point midway the fuel supply pipe. There is also provided the bypass passage that connects the three-port solenoid valve to the air intake system located downstream from the engine throttle valve. A control means is then provided for controlling the position of the three-port solenoid valve so as to direct gaseous fuel toward the bypass passage side only during starting of the engine, while, during acceleration, actuating the auxiliary injector so as to correct the amount of gaseous fuel supplied.
This arrangement ensures a smooth operation of the three-port solenoid valve, providing communication at one time with the fixed venturi side of the gas mixer and at another time with the bypass passage side, thereby allowing the gaseous fuel to flow smoothly. While ensuring a smooth flow of gaseous fuel, the arrangement directs the fuel toward the bypass passage side during, for example, starting the engine. This eliminates a situation, in which the gaseous fuel is hard to discharge because of a slow flow rate at the fixed venture, thus improving startability.
Japanese Patent Laid-open No. Hei 9-21355 discloses a fuel supply system provided with a regulator that reduces the pressure of the fuel gas to a supply pressure, an oxygen concentration sensor that detects concentration of oxygen in an exhaust gas, an idle state detecting means that detects an engine running in an idle state, and a supply pressure regulating means that regulates the supply pressure when the idle state is detected and, at the same time, an output value of the oxygen concentration sensor falls outside a predetermined range. According to this fuel supply system, the oxygen concentration sensor functions to detect an air-fuel ratio during the idle state and, if it detects a ratio deviating from a predetermined target value, the system corrects the air-fuel ratio, either rich or lean, by means of the fuel gas supply pressure.
The conventional fuel supply system for gaseous fuel engines, such as this one, is designed to maintain a central value for air-fuel ratio correction when the air-fuel ratio during idling gradually deviates due to deterioration with time or the like. The amount of fuel gas supplied is greatly affected by variations in the pressure at a supply port of the fuel gas. Especially when the target idle speed varies greatly causing the flow rate at an ISC valve to increase, the pressure of the supply port changes, thus affecting the air-fuel ratio, which results in the engine speed fluctuating or other effects. In the worst case, the fuel gas becomes excessively rich and the engine speed does not increase, thus diffusing an ISC valve correction amount.
The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide an engine fuel control device and an air-fuel ratio control method during idling that is capable of maintaining a stabilized air-fuel ratio during idling and obtaining a stabilized engine speed.
To achieve the foregoing object, an engine fuel control device according to the present invention is basically provided with a fuel supply means that supplies an engine with a fuel, a mixture ratio determination means that establishes a mixing ratio of a mixture of the fuel and air, a mixture introduction means that introduces the air-fuel mixture, whose mixing ratio is established, a first throttle valve that is disposed in an air intake pipe of the engine, a bypass passage that bypasses the first throttle valve, and a second throttle valve that is disposed in the bypass passage. This engine fuel control device is characterized in that it is further provided with a target speed setting means that sets a target engine speed during idling, a throttle valve opening control means that controls the opening of the second throttle valve so as to maintain the target engine speed, a control factor setting means that sets a factor so as to control the opening of the second throttle valve, a capturing means that captures a change in a state of the factor, and a control means that controls the mixture ratio determination means based on the change in the state of the factor captured by the capturing means.
An idling air-fuel ratio control method according to the present invention controls the opening of a throttle valve disposed in a bypass passage, thereby maintaining a target engine speed during idling. The control method is characterized in that it sets a factor for controlling the throttle valve opening, captures a change in a state of the factor, and controls, based on the change in the state of the factor captured, a mixture ratio determination means that determines a mixture ratio of fuel and air.
According to the engine fuel control device and the idling air-fuel ratio control method configured as described in the foregoing paragraphs, when there is a change in the idling target speed, it changes the ISC valve opening accordingly. The change accordingly changes the pressure in a venturi chamber and the amount of inflow of the fuel mixture gas. As a solution to the aforementioned situation, the air-fuel ratio can be prevented from becoming excessively rich or excessively lean by controlling in advance an air bleed valve in accordance with the amount of change in the target speed. The idle speed may also deviate from the target speed due to a disturbance on the engine, at which time, the ISC valve opening also changes. The change in the ISC valve opening represents a change in a basic amount and an ISC valve feedback amount. The air bleed valve is then controlled in accordance with this change, which makes it possible to prevent the air-fuel ratio from becoming excessively rich or excessively lean in the same manner as when there is a change in the target speed.
In a preferred embodiment of the engine control device according to the present invention, the mixture ratio determination means is provided with a means that supplies the fuel supply means with fuel and a means that supplies the fuel supply means with air. It is characterized in that it determines a supply ratio of these two supply means.
In the preferred embodiment of the engine control device according to the present invention, the control factor setting means sets, among other control factors it is to set, a basic opening of the throttle valve predetermined for maintaining the target speed.
In the preferred embodiment of the engine control device according to the present invention, the control factor setting means sets, among other control factors it is to set, an opening as calculated through a feedback control performed for making for any deviation between the engine speed and the target speed.
In the preferred embodiment of the engine control device according to the present invention, the control factor setting means sets, among other control factors it is to set, a value storing an opening as calculated through a feedback control performed for expediting convergence of the deviation between the engine speed and the target speed.
In the preferred embodiment of the engine control device according to the present invention, the capturing means captures a change in the state of the factor from the amount of change in the target speed.
In the preferred embodiment of the engine control device according to the present invention, the amount of change in the target speed is detected based on a deviation between the current engine speed and the target speed.
In the preferred embodiment of the engine control device according to the present invention, the capturing means captures a change in the state of the factor from the amount of change in the predetermined basic opening of the throttle valve.
In the preferred embodiment of the engine control device according to the present invention, the capturing means captures a change in the state of the factor from the amount of change in the opening as calculated through the feedback control.